Solar eruptions in early 2005 led to a substantial barrage of charged
particles on the Earth's atmosphere during the 16–21 January
period. Proton fluxes were greatly increased during these several days
and led to the production of HO<sub>x</sub> (H, OH, HO<sub>2</sub>) and
NO<sub>x</sub> (N, NO, NO<sub>2</sub>), which then caused the
destruction of ozone. We focus on the Northern polar region, where
satellite measurements and simulations with the Whole Atmosphere
Community Climate Model (WACCM3) showed large enhancements in
mesospheric HO<sub>x</sub> and NO<sub>x</sub> constituents, and
associated ozone reductions, due to these solar proton events
(SPEs). The WACCM3 simulations show enhanced short-lived OH and HO<sub>2</sub> concentrations throughout
the mesosphere in the 60–82.5° N latitude band due to the
SPEs for most days in the 16–21 January 2005 period, somewhat higher in abundance than those observed by the Aura Microwave Limb Sounder (MLS).
These HO<sub>x</sub> enhancements led to
huge predicted and MLS-measured ozone decreases of greater than 40 %
throughout most of the northern polar mesosphere during the SPE
period. Envisat Michelson Interferometer for Passive Atmospheric
Sounding (MIPAS) measurements of hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>)
show increases throughout the stratosphere with highest enhancements
of about 60 pptv in the lowermost mesosphere over the 16–18 January
2005 period due to the solar protons. WACCM3 predictions indicate
H<sub>2</sub>O<sub>2</sub> enhancements over the same time period of
about three times that amount. Measurements of nitric acid
(HNO<sub>3</sub>) by both MLS
and MIPAS show an increase of about 1 ppbv above background levels in
the upper stratosphere during 16–29 January 2005. WACCM3 simulations
show only minuscule HNO<sub>3</sub> increases (<0.05 ppbv)
in the upper stratosphere during this time period. Polar mesospheric enhancements of NO<sub>x</sub> are
computed to be greater than 50 ppbv during the SPE period due to the
small loss rates during winter. Computed NO<sub>x</sub> increases,
which were statistically significant at the 95 % level, lasted about
a month past the SPEs. The SCISAT-1 Atmospheric Chemistry Experiment
Fourier Transform Spectrometer NO<sub>x</sub> measurements and MIPAS
NO<sub>2</sub> measurements for the polar Northern Hemisphere are in
reasonable agreement with these predictions. An extremely large ground
level enhancement (GLE) occurred during the SPE period on 20 January
2005. We find that protons of energies 300 to 20 000 MeV,
associated with this GLE, led to very small enhanced lower
stratospheric odd nitrogen concentrations of less than 0.1 %
and ozone decreases of less than 0.01 %.